The preparation of urea from ammonia and carbon dioxide occurs in two steps, via the intermediate product ammonium carbamate and under the usual conditions of temperature and pressure, the conversion of ammonia and carbon dioxide into ammonium carbamate according to the exothermic reaction: EQU 2NH.sub.3 + CO.sub.2 .revreaction. NH.sub.2 COONH.sub.4 + a kcal
is practically complete, while the conversion of the ammonium carbamate (NH.sub.2 COONH.sub.4) into urea according to the endothermic reaction: EQU NH.sub.2 COONH.sub.4 .revreaction. CO(NH.sub.2).sub.2 + H.sub.2 O - b kcal
is only a partial one.
In practice the urea synthesis is carried out in the presence of an excess quantity of ammonia in order to increase the conversion to urea to a higher degree. The solution obtained during the synthesis therefore contains, in addition to urea and water, unconverted ammonium carbamate and free ammonia, and these materials should be removed from the solution before it can be concentrated and processed into an acceptable end-product.
According to a known process as described in Dutch Patent Specification No. 101,446 and U.S. Pat. No. 3,356,723 this is done by subjecting the urea synthesis solution that remains after urea synthesis under pressure to a stripping treatment with gaseous carbon dioxide, in which the ammonium carbamate is decomposed into ammonia and carbon dioxide, with the expulsion of a gas mixture consisting of ammonia, carbon dioxide and water vapor. The decomposition of the ammonium carbamate and the expulsion of the ammonia and carbon dioxide are carried out in the stripping process by reducing the partial pressure of one of the two reactants with the aid of the stripping gas and by the addition of heat. In order to accomplish this the urea synthesis solution is made to flow in a downward direction as a thin film along the internal walls of a number of heated vertical tubes and in a countercurrent with the stripping gas. In this known process therefore the amount of heat necessary for the decomposition of the ammonium carbamate is supplied during the expulsion from the urea synthesis solution of part of the reactants liberated by the decomposition, the expulsion being carried out with the aid of the stripping gas. Stripping is effected counter-currently in order to achieve as favorable an expulsion efficiency of the ammonia and carbon dioxide as possible which are the unconverted reactants. The disclosure of U.S. Pat. No. 3,356,723 is incorporated herein by reference to the extent necessary to understand the process as described herein, it being understood that the present invention represents an improvement over the process described in said U.S. patent.
In this known process however there is a risk of undesirable flooding phenomena occurring for, in the case of flooding, the liquid film flowing downward in the tubes is, as it were, peeled from the wall by the ascending gas and as a result the film becomes detached from the wall which may cause the formation of liquid plugs which occlude the affected tubes. Flooding phenomena usually occur in the presence of high gas and liquid loads and, hence, occur first in the top part of the tubes because here these loads are highest. They result from the application of the counter-current principle and their occurrence is further promoted by the addition of heat via the tube wall simultaneously with the stripping, since this causes the liberated heat to flow in a direction away from the wall. In this connection, in order to avoid flooding, only a limited heat flux can be allowed for a given tube diameter and at given gas and liquid loads, hence placing practical limitations on the process. The present invention provides a method of eliminating this by adding sufficient heat to the urea synthesis solution to cause decomposition of a major portion of the ammonium carbamate prior to contact with the stripping gas.